Process for the cleaning of equipment



ICC

United States Patent Patented Apr. 22, 1969 US. Cl. 252-147 6 ClaimsABSTRACT OF THE DISCLOSURE trode, and is maintained at a value below 280mv. by adjusting the quantity of stannous salt, whereby reduction offerric ions to ferrous ions by steel in the equipment, and consequentcorrosion, is prevented. The solution may also contain a complexingagent wh i c hfpim qlgblwmplexes with fernc ions, such as ammoniumbifluoride", 51TH arreraaesfia'tfaim soluble complexes with ferrousions. Scale such as mill scale is dissolved without corrosiop of theiron in the metal even when the solution is circu lated at a high flowrate and when it is heated to elevated temeperatures.

The invention relates to a process for the cleaning of equipmentconsisting wholly or partially of metal.

In the industry use is frequently made of equipment which, as aconsequence of the medium with which it is in contact, has to beregularly cleaned. Examples of such equipment are steam boilers, pipes,reactors, heat exchangers, pumps, valves, distillation columns etc. Withinternal cleaning it is in some cases possible to carry out thiscleaning operation by a mechanical method, such as by scaling, scraping,scouring, polishing etc. In order to carry out these mechanicaloperations, however, it is necessary to dismantle the equipment,whereupon the component parts may be cleaned separately.

It will be obvious that mechanical cleaning involving dismantling istime-consuming and therefore very expensive. It is also sometimesdifficult and cumbersome to clean satisfactorily small spaces, which aredifficult of access, such as bubble caps and rather long, in particularthin, pipes.

In view of the drawbacks involved in mechanical cleaning, attempts arebeing made to achieve the desired cleaning with chemical agents of sucha type that the dismantling of the installation to be cleaned can becompletely or substantially completely omitted. This object is achievedby the use of chemical cleaning agents which can be pumped through theequipment. This chemical cleaning method not only carries the advantageof a saving of time and labour but also improves the thoroughness of thecleaning.

The liquid cleaning agents used for this purpose are' usually inorganicor organic acids. According to the nature of the contamination and ofthe metal to be cleaned use is made in the chemical cleaning process ofinhibited or uninhibited acids, such as citric acid, sulphamic acid,phosphoric acid, nitric acid, sulphuric acid and hydrochloric acid. Forthis purpose the use of hydrochloric acid is usually preferredespecially due to its low cost, ease of handling and great capacity fordissolving many deposits and corrosion products. An inhibitor is usuallyadded to the acid in order to counteract as far as possible thecorrosive effect of these acids on the equipment to be cleaned. Examplesof such inhibitors include hexamethylene tetra.- mine, alkaloids andquaternary ammonium bases.

In the practical application of said chemical cleaning methods, however,serious difliculties have been encountered. When said acids are used infact corrosion occurs even in the presence of inhibitors-as a result ofthe formation of ferric ions originating from the iron-containingequipment to be cleaned and/or from the impurities containing ironcompounds. This corrosion can even occur to such a large degree thatafter cleaning pitting and scouring are encountered in the metal of thecleaned equipment. This phenomenon occurs particularly with unalloyedsteels and steels of low alloy, which are frequently used for example insteam boilers. The corrosion causes the formation of rough metalsurfaces which in steam boilers lower the flow rate of the Water in theevaporator tubes, with the result that a reduction of the heat transferand an increase in the wall temperatures in the installation may occur.

When combinations of a nobler and a less noble metal are present in theequipment to be cleaned, an intensified corrosive attack on the lessnoble metal is found to occur. It is indeed possible to reduce thisintensified corrosion, for example by blanking off or removing thenobler metal of the metal combination from the apparatus before thecleaning operation, but it will be obvious that in practice this is atleast inconvenient and often impracticable.

To this the following must be added. The corrosive attack appears toincrease considerably when there is an increase in the flow rate of thecleaning acid. Thus, when the flow rate of the cleaning acid is raised,e.g. to above 0.5 m./sec., the corrosion of the metal to be cleanedincreases greatly. This means that such flow rates will have to beavoided as much as possible in order to avoid damage to the installationwhich is to be cleaned. This, however, may not always be possible whenthe equipment to be cleaned is of a complicated design. In localconstrictions within a tube system for instance, excessive rates offiowmay easily occur, with the result that the corrosion at such a point canassume serious dimensions.

In view of the important advantages of chemical cleaning with respect tomechanical cleaning the first-named method is used on a large scale inpractice in spite of the above-mentioned drawbacks. In order, however,to keep the degree of corrosive attack to some extent within acceptablelimits it is often necessary to sluice the cleaning acid one or moretimes during the cleaning operation, and to replace it by fresh acid.This is an attempt to keep within certain limits the content of ferricions in the cleaning acid which are responsible for the corrosion. Evenwhen such a measure is taken the occurrence of corrosion is in practiceoften unavoidable. It thus appears that, when steel comes into contactwith an inhibited hydrochloric acid bath which is in motion and containsferric compounds, at a temperature as low as 20 C., 50% of the ferricions are reduced within 2 hours to ferrous ions at the expense of theequivalent amount of steel, which goes into solution, or in other words,corrodes. At 50 C., a normal temperature for cleaning steam boilers, forexample, this degree of corrosion is reached even within an hour. Sinceit may take some hours to fill a fairly sizable steam boiler withcleaning fluid by means of a circulation pump, strong corrosive attackhas already taken place before it is possible to proceed to sluicing thecleaning acid containing ferric ions.

It can be seen, therefore, that serious corrosion problems areinvariably encountered in practice in methods of chemical cleaning ofequipment by means of a cleaning acid-even when cumbersome measures aretaken, like the replacement of the cleaning acid by fresh acid.

According to the present invention it has now been found that the abovedrawbacks can be wholly avoided if a stannous salt, which is soluble inthe cleaning acid, such as stannous chloride, is added to said inhibitedcleaning acid and thereby inherently forms stannous upon dissolving.When a cleaning fluid of this type is used the corrosive attack--even athigh rates of flow of the cleaning acid-is found to be whollysuppressed, while if an inhibitor is used any tin which happens to beprecipitated does not impair the corrosion resistance of the metal to becleaned either.

The invention therefore relates to a process for the cleaning ofequipment consisting wholly or partially of metal which comprisestreating the equipment with a cleaning fluid comprising an aqueoussolution of an inhibited inorganic or organic acid which, in addition,contains a stannous salt soluble in this solution.

Stannous chloride is preferably used as stannous salt soluble in thecleaning acid and therefore the source of stannous ion and inhibitedhydrochloric acid as cleaning acid. Depending on the quantity and thenature of the contaminations to be removed, concentrations of thehydrochloric acid of between 0.25% by weight and 15% by weight, appearto be very satisfactory. As to the quantity of the stannous chloride, independence on the concentration of ferric ions in the cleaning acid, anddepending upon the redox potential as will be discussed hereinbelow,quantities of between 1 and 30 g. per litre of cleaning acid whichtherefore corresponds to a stannous ion concentration of between 0.53and 15.8 grams per liter of against corrosion-can now be wholly omitted.The flow rate of the cleaning acid can be raised to high values withoutthe corrosive attack exceeding acceptable limits. In order to gain thefull profit from the present invention flow rates for the cleaning acidof at least 0.2 m./sec., and preferably of between 0.5 and 3 m./ sec.will be used, so that a rapid and effective cleaning can be obtainedwhile avoiding a degree of corrosion of any substance.

The process according to the invention also provides protection againstcorrosive attack in equipment which contains combinations of noble andless noble metals, a protection which formerly could be only obtained byremoving or blanking off the nobler metal concerned before the cleaningoperation. It was thus established that by the use of the cleaning acidaccording to the invention the corrosive attack on the following metalsor combinations of metals was wholly or substantially wholly suppressed.

Mild steel Carbon steel 50-60 1% Cr-Vz Mo-steel 5% Cr-Vz Mo-steel CopperBrass Aluminum-brass Tin-bronze C-upronickel -10 and 70-30 InconelNionel Nickel Monel Hastelloy 13 chromium steel 17 chromium steelStainless steel, type 18% Cr-8 to 10% Ni The advantages of the presentinvention are evidenced very clearly in the cleaning of the frequentlyused steels which are unalloyed or alloyed with chromium and/or nickeland/or molybdenum, in particular steels alloyed with 15% of chromium and04-06% of molybdenum. In particular steel with 1% of chromium and 0.5%of molybdenum displays the phenomenon that the hydrogen which formsduring the corrosive attack diffuses in the metal and gives rise tohydrogen brittleness. High pressure steam pipes are often manufacturedfrom this material, and the disadvantage is frequently experienced withthe conventional chemical cleaning methods that the corrosive attack isaccompanied by hydrogen brittleness in the cleaned material, with theresult that the latter easily fractures due to its brittleness. Thisdisadvantage also is wholly obviated by using the process according tothe invention.

It is also to be noted that a special aspect of the present invention isthe possibility of maintaining the composition of the cleaning acid, bya simple continuous electrometric control of the cleaning fluid, at sucha level that throughout the entire cleaning process the certainty existsthat no corrosion, or substantially no corrosion, will occur. Thus, whenmeasured with a platinum electrode against a saturated calomel-KCIelectrode, the redox potential in the presence of a corrosive quantityof ferric ions in inhibited hydrochloric acid of 10% concentrationamounts to 200 to 400 mv., de ending upon the quantity of ferric ions;in the presence'of a quantitiy of stannous ions, su cient to counteractcorrosion, the redox potential is below 280 mv. and in practice it isadvisable to work below mv. By means of this electrometric control it isnow possible to keep the re d 9gr p otemial @ianarrqi lmtsmrmmsai in toa P pweiiiifs iu by i itsihe. 2it ..in2t2.922 L of the l i l jilllt-QililliiOhQfiQ.Ifih q y at2iii i li 222 The temperature at which thecleaning operation is carried out is usually the normal ambienttemperature. In the classical chemical cleaning process it was generallynecessary to avoid as much as possible temperatures higher than theambient temperature in view of the intensified corrosive attack atelevated temperatures. The cleaning fluid used according to theinvention is characterized in that also at elevated temperatures thereis no corrosive attack. The upper limit of the temperature to be used isonly determined by the stability of the inhibitor in the inhibited acid,and this limit generally lies around 70 C. The present invention,therefore, now provides for the first time the possibility of carryingout the cleaning operation conveniently with a cleaning fluid at anelevated temperature, i.e. at a temperature of up to 70 C.

In a preferred embodiment of the present invention a cleaning fluid isused which in addition to an inhibited inorganic or organic acid and asoluble stannous salt, contains a complexing agent specific for ferricions. It has been found that in that case the amount of stannous salt tobe used can be considerably reduced; amounts of less than of the amountof stannous salt originally applied have been found to be suflicient forsuppressing or substantially reducing the corrosive attack of the metalcontrol of the redox potential as explained above and illustrated forthe combination stannous chlorideammoniumbifluoride in Example IV.

EXAMPLE I The effect of the cleaning fluid to be used according to theinvention can be seen from the data described in the following. The testresults given in this example were obtained from tests which werecarried out as follows.

Test plates from unalloyed steel (dimensions 60 x x 1.5 mm.), from whichplates the grease had been removed and which had been scoured bright,were exposed in duplicate in beakers containing 0.5 l. of inhibitedironfree hydrochloric acid. As inhibitor use was made of a commerciallyavailable inhibitor based on quaternary ammonium bases in aconcentration of 0.2% by weight.

The test plates were placed vertically on their long sides in smallglass racks in the beakers. Beside each exposure in a stationary mediumtest plates were exposed in a moving medium having a rate of 0.2 1111.per second, which movement was obtained by means of an electricallypowered glass stirrer. The plates were exposed for four periods, viz 8,8, 8 and 24 hours, in the same fluid, unless otherwise stated. Aftereach exposure the plates were weighed and examined for signs of pitting.

TABLE I [Medium: 10% HCl+inhibitor+10 g. of Fe+++ per litre; 20 0.]

Exposure Fluid stationary Rate of fluid 0.2 in. per sec.

Reduction v Reduction Period Hrs. of weight Remarks of weight Remarks111 mg. in mg.

8 168 Surface slightly scoured, edges 1, 258 Distinct scouringparticularly {(43 distinctly corroded. 1g; edges heavily corroded. 8 162IIIII"'IIIIII 94 284 34 212 33 9 24 5.3% Corrosion visibly increased. 71, 482 Total over 48 742 1' 324 Surface 2. 3,685 mm. Surface a=3,685 mm.Surface b=3,595 mm. Surface b=3,685 mm.

TABLE II [Mediu.m: 10% HCH-inhibitor+20 g. of Fe per litre; 20 C.]

Exposure Fluid stationary Rate of fluid 0.2 m. per sec.

Mild Reduction Reduction Period Hrs. steel of weight Remarks of weightRemarks plate 1n mg. in mg.

618 Slight scouring, edges 2. 548 Rather heavy scouring of the surgggdistinctly corroded. 2, face; edges heavily corroded. 429 173 360 50 35350 510 Corr of the edges 61 332 intensified. 64 1, 920 2, 852 1, 769 514Surface a=3,745 mm. Surface b=3,735 mm.

Surface a=3,745 mmJ Surface b=3,675 mm.

to be cleaned. This is more fully illustrated in Example IV.

The complexing agent should be specific for ferric ions in that itshould not form a complex with the ferrous ions. This is essential forthe purpose of the invention and another essential feature of thepresent embodiment is that an amount of the soluble stannous saltbe it aconsiderably reduced amount-should always be present.

Complexing agents which may be used include phosphoric acid orhydrofluoric acid or its salts of which ammoniumbifluoride '(NH HF hasbeen found especially suitable. The relative amounts of stannous saltand complexing agent can easily be fixed at the desired level by Whenthe inhibitor applied is replaced by other conventional inhibitorscorresponding results were obtained, as also with the use of I-lCl in a5% concentration.

From the above Tables I and II the following can be deduced:

(a) The corrosion increases by a factor of about 4 to 1 TABLE III tionspossible, viz at high flow rates of the cleaning fluid which moreoverhas a temperature of 50 C.

EXAMPLE II Exposure tests of (a) rotating steel discs, mild steel(Insufliciency oi Sn++) (Very slight excess Sn++) Exposure Medium:Medium:

% E01 plus inhibitor+10 g. oi Fe=/l. 10% E01 plus inhibitor+10 g. Fe ll.

+6 g. of SnC1z.2H2O/l. g. of Sn0l .2H O/l. Rate 0.2 m./sec.; temp., 200. Rate 0.2 m./sec.; temp., 20 C.

Period Hrs. Plate Reduction 0! Remarks Reduction 0! Remarks weight, mg.weight, mg.

873 Distinct scouring, edges 70 870 slightly corroded. 7(5) 71 71 6 8 2810 34 9 24 }Nc corrosion visible.

Surface a=3,520 mm. Suriace b=3,520 mm.=

(Excess Sn++) (Excess Sn++) Exposure Medium: Medium:

10% H01 plus 1nh1bitor+l0 g. of Fe ll. 10% 1101 plus inhibitor-H0 g. Fo/l.

g. of 81101:.21120/1. 50 g. of SIlCl2.2HzO/l. Rate 0.2 m./sec.; temp.,20 0. Rate 0.2 m./sec.; temp 2F 0.

Period Hrs. Plate Reduction 0! Remarks Reduction 0t Remarks weight, mg.weight, mg.

72 73 75 74 3 2 2 4 2 1 1 1 a }No corrosion visible g }No corrosionvisible.

Surface a=3,620 mm. Surtace b=3,670 mm.

Surface a=3,700 mm. Surface b=3,580 mm.

first period of exposure.

It can also be seen from the above Table III that when with mill scale;(b) welded steel pipe, mild steel, inside diameter 19 mm., through whichthe fluid was pumped.

Medium: 10% HCl+l0 g. of Fe+++ per litre-l-inhibitor +small excess ofSnCl .2H O.

Temperature: C. (:L2 C.) for all tests.

Exposure time: 15% hours.

TABLE IV Rate of fluid Ex sed Reduction Reduction Corrosion Materialmild steel with respect to su ace in of weight of weight atter expressedin Remarks the metal, in mm. in mg. correction for mm./per

m/sec. scale in mg. annum 8. Disc with mill S0810 1- 1 10, 7 5 200 1- 3swig entirely smooth and undam- 1 2. c5 10, 708 971 325 2.1 D6. Do i 3.0 10, 708 958 312 2. 0 D b. Welded pipe 400 mm. in lengt 2. 3 654 2. 0Do. b. Welded pipe 2,000 mm. in length 2 3 Do.

1 Average.

It is seen that under conditions which would lead to heavy corrosionwith the known chemical cleaning methods, no or substantially nocorrosion occurs.

EXAMPLE III This example illustrates the cleaning of metal combinatronswith the process of the invention. Thest plates of difierent metals werecombined with test plates of mild steel and the metal combination wasexposed at 20 C. during 16 hours to cleaning acids of the compositionand under the conditions set out in the following Table V.

TABLE V H l 101%,h M230 +125 3' Fe' /l plus 10% HCI+10 g. Fe /l. plusinhibitor; temp.- 0.; rate 0.2 m./sec.

Red. in Red. in Metal combination weight Remarks weight Remarks in mg.in mg. Mild Steel 3% }Steel has slight copper scale 3 }Metal unchanged.

:2 Steel has no copper scale, but brass shows dark discolouration i Do.:1; }Steel has no copper scale, but Al-brass has a black scale g Do. ig}Steel has greenish scale and Crsteel shows dark discolouration; alterscouring g Do. 205 slight pitting visible. 2

}Stee has greenish scale which can be easily removed 1 Do.

It will be seen that also in the case of combinations of Exposure ofmild steel test plates in ferrous and ferric ions a noble with a lessnoble metal which present serious corcontaining inhibited hydrochloricacid rosion .difiicultiFs in the known chemical cleaning Composition ofthe cleaning fluid at the beginning of the rosion is essentiallysuppressed when applying the process 20 test mm of the invention. 1 0 nHe! 7 57 1 re 0, EXAMPLE IV Inhibitor 0.3%, This example illustrates theuse of a complexing agent vAmmoniumbifluoride (NH HF 10 grams, specificfor ferric ions as additive in the cleaning fluid Stannous chloride(SnCl .2H O), 1.0 gram. and the considerable saving of stannous saltobtained Temperature 50 C.; rate of fluid 0.2 m./sec.

Addition of Redox Time after start 01 test run SnCh-2Ha0 potentialRemarks in grams in mv.

0 hr Dissolved 10.4 g. mill scale present on mild steel plate, 1 hr. 15min. 230

7 mild steel plates placed in bath:

1 hr. 30 min 1. During the whole of the exposure the redox potentialstayed in the range in which (in the presence of the inhibitor) nocorrosive attack takes place.

2. The total consumption of SnCia-2Hz0 was 1.15 gram which is about 10%of the amount required in the absence of the ferric ion complexingagent.

thereby with full maintenance of the eificiency of the The averagecorrosive attack of the test plates is 0.24 cleaning method according tothe present invention. mm. a year. None of the test plates showedpitting.

Influence of fluoride on the redox potential of inhibited I clam:

1. In a process for the cleaning of metallic equipment gg ggg g g zi gvarymg rams of ferrous Sulphate constructed essentially of ironcontaining metal by means of a recirculating aqueous corrosion inhibitedacidic clean (A) 1 litre HCl 7.5%, 0.3% inhibitor ing solution, theimprovement which comprises the steps (B) 1 litre I-ICl 7.5%, 0.3%inhibitor, 30 gram ammomf umbifluoride (1) contacting the surface of theequipment to be cleaned with 'a circulating aqueous inhibited acidiccleaning solution comprising (a) from about 0.25 to Further additions asindicated. Both test runs at 50 C.; rate of fluid 0.2 m./ sec.

A. Without fluoride B. With fluoride 15 weight percent of an acidselected from the group consisting of citric, sulfamic, phosphoric,nitric, sulfuric and hydrochloric acids, and (b) stannous ion in aconcentration of from 0.53 to 15.8 grams per liter, the stannous ionbeing derived from a soluble stannous salt, and

Totally added: 5 grams ferrous, 1.3 grams ferric, 0.22 gram SnCl .2H O,i.e. less than 10% of the amount required in the absence of fluoride tokeep the fluid free from ferric ions.

The bath without fluoride is corrosive.

The bath with fluoride is non-corrosive.

(2) adding a soluble stannous salt to the recirculating cleaningsolution in a quantity sufficient to maintain the redox potential of thesolution below 280 mv. as measured by a platinum electrode against asaturated calomel-KCl electrode, whereby the ferric ion con tent of thesolution is controlled.

2. The process of claim 1, wherein suflicient soluble stannous salt isadded to maintain the redox potential below 170 mv.

3. The process of claim 1, wherein the acid is hydrochloric acid.

4. The process of claim 1, wherein the soluble stannous salt is stannouschloride and is present in a concentration of from 1 to 30 grams perliter of solution.

5. The process of claim 1, wherein the corrosion inhibitor is selectedfrom the group consisting of hexamethylene tetramine, alkaloids, andquaternary ammonium bases and the inhibitor is present in an amount offrom about 0.2 to 0.3 weight percent.

6. The process of claim 1, wherein the circulating cleaning solution hasa rate of flow through the equipment of at least 0.2 meter per second.

7. The process of claim 1, wherein the circulating cleaning solution hasa rate of flow through the equipment in the range of between 0.5 to 3meters per second.

8. The process of claim 1, wherein the temperature of the cleaningsolution is between ambient and 70 C.

9. The process of claim 1, wherein the equipment is constructed of ametal selected from the group consisting of steel and steel alloys withat least one of the alloyed metals selected from the group consisting ofchromium, nickel and molybdenum.

10. The process of claim 9, wherein the equipment is constructed of asteel containing from about 1 to 5 percent chromium and from about 0.4to 0.6 percent molybdenum.

11. In a process for the cleaning of metallic equipment constructedessentially of iron containing metal by means of a recirculating aqueouscorrosion inhibited acidic cleaning solution, the improvement whichcomprises the steps of (1) contacting the surfaces of the equipment tobe cleaned with a circulating aqueous corrosion inhibited acidiccleaning solution comprising (a) from about 0.25 to 15 weight percent ofan acid selected from the group consisting of citric, sulfamic,phosphoric, nitric, sulfuric and hydrochloric acids, (b) stannous ion ina concentration of from 0.053 to 15.8 grams per liter, the stannous ionbeing derived from a soluble stannous salt, and (c) a complexing agentfor ferric ions selected from the group consisting of hydrofluoric acidand ammonium bifluoride, said complexing agent being present in anamount soluble in the acidic solution; and

(2) adding soluble stannous salt to the recirculating cleaning solutionin a quantity sufficient to maintain the redox potential of the solutionbelow 280 mv. as measured by a platinum electrode against a saturatedcalomel-KCl electrode, whereby the ferric ion content of the solution iscontrolled.

12. The process of claim 11, wherein the ferric ion complexing agent isammonium bifluoride.

13. The process of claim 11, wherein both the soluble stannous salt andthe ferric ion complexing agent are added to the recirculating cleaningsolution in an amount suflicient to maintain the redox potential belowmv.

14. The process of claim 11, wherein the redox potential is maintainedat a value below about 170 mv. by the addition of stannous chloride andthe ferric ion complexing agent.

15. An aqueous based cleaning composition for cleaning equipmentconsisting essentially of iron-containing metal consisting essentiallyof water, 0.25 to 15 weight percent of an acid selected from the groupconsisting of citric, sulfamic, phosphoric, nitric, sulfuric, andhydrochloric, from 0.053 to 15.8 grams per liter of stannous ion derivedfrom a soluble stannous salt, from about 0.2 to 0.3 weight percent of acorrosion inhibitor selected from the group consisting of hexamethylenetetramine, alkaloids, and quaternary ammonium bases, and a ferric ioncomplexing agent selected from the group consisting of hydrofluoric acidand ammonium bifluoride present in an amount soluble in the cleaningsolution.

16. An aqueous based cleaning composition for cleaning equipmentconsisting essentially of iron-containing metal consisting essentiallyof water, 0.25 to 15 weight percent hydrochloric acid, from 0.1 to 30grams per liter of stannous chloride, from about 0.2 to 0.3 weightpercent of a corrosion inhibitor selected from the group consisting ofhexamethylene tetramine; alkaloids, and quaternary ammoniumbass, andammoninum bifiuoride as a ferric complexing agent in a concentration offrom about 10 grams to 30 grams per liter.

References Cited UNITED STATES PATENTS 1,460,395 7/1923 Vogel 252-1481,678,776 7/ 1928 Gravell et al 252-147 1,773,247 8/1930 Williams252-147 2,927,871 3/1960 Mancke et a1. 156-19 3,033,795 5/1962 Brevik252-146 LEON D. ROSDOL, Primary Examiner.

W. SCHULZ, Assistant Examiner.

US. Cl. X.R.

